[AGIPD] Split AGIPDCtrl function for clear code and adding tests later.

Based on changes in MR : https://git.xfel.eu/detectors/pycalibration/-/merge_requests/652

This MR is a preparation for the next MR for adding unit tests https://git.xfel.eu/detectors/pycalibration/-/merge_requests/646

Description

• Separate get_gain_setting() and get_acq_rate() into two different functions which read the gain_settings and acquisition rate using two different methods.

• Small refactors:

  • correct hint typing for get_gain_mode()
  • correct the doc string for get_num_cells()
  • correct the doc string for AGIPDCtrl()

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

Checklist:

• I added tests where appropriate. In a separate MR: https://git.xfel.eu/detectors/pycalibration/-/merge_requests/646

Reviewers

@calibration